Fluorine-containing copolymers comprising a vinyl monomer unit having hydrocarbon type functional group are described in JP-A-1-103670, JP-A-8-41131, etc. and it is known that they are useful as crosslinking coating resins when used in combination with a curing agent. Also a fluorine-containing copolymer prepared by copolymerizing perfluorobutenoic acid or a vinyl ether having a fluorine-based functional group is known as a fluorine-containing copolymer comprising a fluorine-containing vinyl monomer.
However a fluorine-containing resin copolymer which comprises a vinyl monomer unit having hydrocarbon type functional group and, is insoluble in tetrahydrofuran (THF) substantially has not been known. In case of a resin being soluble in THF, in order to realize solvent resistance, a combination use of a curing agent capable of forming a crosslinked structure has been essential. On the other hand, from the aspect of use as a molding material, when using the copolymer together with a curing agent, it was necessary to control molding and curing reaction in a very narrow temperature range, and thus heat-molding was very difficult substantially.
Also since a monomer having a fluorine-based functional group has a good reactivity with fluoroolefin such as tetrafluoroethylene (TFE), it can introduce a functional group into a melt-moldable resin insoluble in THF such as ethylene-tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene/hexafluoropropylene copolymer (FEP) and tetrafluoroethylene/perfluoro(vinyl ether) copolymer (PFA). However a preparation process of the monomer ranges over multiple stages and is disadvantageous economically.
The present inventors have found that a hydrocarbon vinyl compound monomer having functional group, particularly a vinyl ether monomer having functional group reacts with a copolymer containing a specific amount of TFE unit and can give a resin insoluble in THF, and thus completed the present invention.